There is a palpable nervousness among Capitol Hill Republicans these days. They are well aware that history has been unkind during midterm elections to the party of second-term presidents: In such elections since the end of World War II, the president's party has lost an average of 29 seats in the House and six in the Senate.

The White House's plans to overhaul Social Security and make deep cuts in domestic spending have heightened the Republicans' anxiety. Recent polling suggests that a cost-of-living "recalculation" that reduces Social Security benefits or a "carve-out" that siphons Social Security payroll taxes into private accounts could be very damaging to the GOP.

But is there really any chance that the Republicans could lose control of the House and Senate in 2006? To seize the Senate, Democrats need a net gain of six seats. Yet, of the 15 Senate seats that the GOP will have to defend in 2006, only six Republicans appear to be even potentially vulnerable: the Tennessee seat that Majority Leader Bill Frist will be leaving, plus those held by George Allen of Virginia, Lincoln Chafee of Rhode Island, Mike DeWine of Ohio, Rick Santorum of Pennsylvania, and Jim Talent of Missouri.

Several of these GOP seats would actually become vulnerable only under very special circumstances. For example, the only Virginia Democrat capable of forcing George Allen to break a sweat appears to be Gov. Mark Warner, but Warner seems more interested in running for president. And although Ohio is certainly a swing state, there's no Democrat now in sight who could give DeWine a real race.

Missouri used to be considered a swing state, but it has increasingly looked quite Republican. And Missouri Democrats have yet to persuade a first-tier challenger to take on Talent.

Democrats are likely to come up with a solid challenger in Pennsylvania, but the state's media markets are extraordinarily expensive, and Santorum has proven to be a tough target in the past. Finally, while Democrats may well have a strong nominee in Tennessee -- Rep. Harold Ford is already in the race -- their recent failure to hold on to open Senate seats in the South should give them real pause.

In the House, where Democrats need a 15-seat gain in 2006, there might not even be 15 Republican-held seats that are truly up for grabs. Although the 2004 presidential election results by congressional district are not yet available, in 2000 George W. Bush carried 229 districts to Al Gore's 206. This suggests that the party ratios in the House fairly well reflect the nation's political leanings.

Currently, 29 House seats appear likely to be competitive in 2006. Eighteen are held by Republicans, 11 by Democrats. Four of those Democrats are expected to have races that are too close to call: John Salazar (CO-03), Melissa Bean (IL-08), Charles Melancon (LA-03), and Chet Edwards (TX-17). Republicans have just two toss-up races -- for the open IA-01 seat being vacated by Jim Nussle and the district represented by Mike Sodrel (IN-09). Of the remaining 16 competitive Republican districts, about half are represented by battle-tested incumbents who have proven that they can win tough races.

So neither the House nor the Senate appears likely to fall into Democratic hands in 2006, even though overhauling Social Security could easily backfire on the GOP, reducing its majorities in both chambers.

But one unpredictable factor should worry the GOP: The closest races generally break, often heavily, in favor of one party. In fact, the winning party generally captures 67 to 89 percent of the Senate races rated as too close to call in the final pre-election ratings in my Cook Political Report. In the House, the range is 53 to 69 percent of toss-ups.

In 1998, for example, Democrats won 86 percent of toss-up Senate races and 69 percent of toss-up House contests. In 2000, Senate Democrats carried 78 percent of such contests; House Republicans took 54 percent. In 2002, Senate Republicans won 71 percent of toss-ups and House Republicans won 69 percent. Last November, Senate Republicans prevailed in 89 percent of such contests and House Republicans in 56 percent.

In short, toss-up House and Senate races tend to break decisively in one direction. And that tendency is especially strong in Senate contests. Because the direction is nearly impossible to predict, GOP strategists can expect plenty of sleepless nights.

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